Exposure apparatus

ABSTRACT

In the exposure apparatus of the present invention, a sealed chamber defined by a first lens and a second lens in an input lens system on a plane of incidence of a fly-eye lens in an illumination optical system is provided. In a gas exchanging step, the impurity gas in the sealed chamber is first exhausted through an electromagnetic valve provided with a check valve and a gas exhaust pipe, using a gas exhaust pump and then, a high-purity nitrogen gas is supplied from a gas bomb through a gas supply pipe and an electromagnetic valve provided with a check valve to the sealed chamber. Using a pressure sensor provided in the sealed chamber, the gas exchanging step is repeated while maintaining an amount of change in pressure in the sealed chamber within a predetermined allowable range, to thereby reduce the concentration of impurities in the gas in the sealed chamber to a target value.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an exposure apparatus which isused for transferring a pattern on a mask to a substrate, such as awafer, in a photolithography process for producing semiconductors,liquid crystal displays, thin-film magnetic heads, etc.

[0002] As exposure apparatuses used for producing, for example,semiconductors, there can be mentioned a projection exposure apparatus,such as a stepper, in which a reticle as a mask is illuminated withexposure light passing through an illumination optical system, tothereby transfer a pattern on the reticle through a projection opticalsystem to a photoresist-coated wafer (or a glass plate), and an exposureapparatus of a proximity type or a contact type-in which the pattern onthe reticle is directly transferred to the wafer, using theabove-mentioned exposure light. In these exposure apparatuses,ultraviolet light, such as an i-line from a super-high pressuremercury-vapor lamp (wavelength: 365 nm), has conventionally been used asexposure light.

[0003] In conventional exposure apparatuses, a series of lenses in theillumination optical system are divided into blocks and fixedly providedin lens barrels. In the illumination optical system, chambers defined byadjacent lenses are sealed by providing sealing materials between lensesand lens barrels. These sealing materials also serve as adhesives forfixing the lenses to the lens barrels. As such sealing materials,silicon-containing materials are generally used. In other words, in thesealed chambers in the illumination optical system, silicon-containingmaterials which serve not only as sealing materials, but also asadhesives are used. It is known that the sealing materials (oradhesives) containing silicon generate an organosilicon gas.

[0004] In conventional exposure apparatuses in which ultraviolet lightis used as exposure light, ozone is produced from oxygen molecules in anatmosphere, in the presence of ultraviolet light. When an organosilicongas is generated from the sealing materials (or adhesives) containingsilicon, the ozone produced from oxygen in the presence of ultravioletlight oxidizes the organosilicon gas and consequently, deposition ofhaze substance, such as silicon dioxide (SiO₂), on the surfaces oflenses is likely to occur. This leads to a lowering of illuminance ofexposure light and a non-uniform distribution with respect toilluminance of exposure light. Because low molecular weight siloxanecontained in the sealing materials (or adhesives) is a cause of thegeneration of organosilicon gas, in order to prevent deposition of SiO₂on the surfaces of lenses in an illumination optical system, recently,materials having a low content of low molecular weight siloxane havebeen used as the sealing materials (or adhesives).

[0005] Thus, in illumination optical systems in conventional exposureapparatuses, sealing materials (or adhesives) which are unlikely togenerate an organosilicon gas, such as materials having a low content oflow molecular weight siloxane, are used. However, such sealing materialsexhibit poor operability due to a prolonged solidification time.Further, even when the content of low molecular weight siloxane in thesealing material is low, liberation of silicon cannot be completelysuppressed, so that an organosilicon gas is generated in a small amountwith a consequence that a small amount of SiO₂ is likely to be depositedon the surface of lens.

[0006] As another example of sealing materials which are unlikely togenerate an organosilicon gas, there can be mentioned non-evaporativetwo-liquid type adhesives. However, such two-liquid type adhesives alsohave poor operability.

[0007] Recently, there has been an increasing tendency to use, asexposure light, excimer laser beams having a short wavelength, such as aKrF excimer laserbeam (wavelength: 248 nm) and an ArF excimer laser beam(wavelength: 193 nm). On the other hand, it is known that when lighthaving a short wavelength, such as excimer laser beams, is irradiated toadhesives containing silicon, silicon is liberated in a large amount.Therefore, it is considered that when excimer laser beams are used asexposure light, deposition of haze substance on the surfaces of lensesoccurs in a wide range in the illumination optical system, so thatcountermeasures for deposition of haze substance have been stronglydesired.

BRIEF SUMMARY OF THE INVENTION

[0008] In view of the above situation, the present invention has beenmade. It is a primary object of the present invention to provide anexposure apparatus in which deposition of haze substance on opticalmembers, such as lenses, in an illumination optical system can besuppressed, to thereby prevent a lowering of light transmittance andlight reflectance of the lenses.

[0009] According to the present invention,-there is provided an exposureapparatus for illuminating a pattern on a mask with exposure lightpassing through an illumination optical system, to thereby transfer thepattern on the mask to a substrate, comprising:

[0010] a sealed chamber provided in an optical path of the exposurelight in the illumination optical system,

[0011] the sealed chamber containing a gas and shielded from a gassurrounding the sealed chamber in the illumination optical system; and

[0012] a gas exchanging device adapted to exchange the gas in the sealedchamber with a predetermined gas.

[0013] In the above-mentioned exposure apparatus, when an inert gas iscontained in the sealed chamber, generation of ozone due to ultravioletlight which is used as exposure light can be avoided, so that even whenan impurity gas, such as an organosilicon gas, is generated from sealingmaterials which are used in optical members (such as lenses) in contactwith the gas in the sealed chamber, deposition of haze substance, suchas SiO₂, on the surfaces of optical members can be prevented. Further,when the gas in the sealed chamber is periodically exchanged with thepredetermined gas, the impurity gas generated from the sealing materialscan be removed. Due to the above two effects, occurrence of haze on thesurfaces of optical members (leading to a lowering of lighttransmittance and light reflectance of the optical members) can besuppressed. As the inert gas contained in the sealed chamber, ahigh-purity nitrogen gas and a rare gas, such as helium, may be used.

[0014] In the above-mentioned exposure apparatus, it is preferred thatthe gas exchanging device comprise:

[0015] a gas exhausting system adapted to exhaust the gas in the sealedchamber;

[0016] a gas supplying system adapted to supply the predetermined gas tothe sealed chamber;

[0017] a pressure sensor provided in the sealed chamber to detect apressure in the sealed chamber; and

[0018] a control system adapted to control an operation of each of thegas exhausting system and the gas supplying system, based on thepressure in the sealed chamber detected by the pressure sensor, tothereby exchange the gas in the sealed chamber with the predeterminedgas.

[0019] In the exposure apparatus having the gas exchanging devicearranged as mentioned above, it is possible to exchange an impurity gasin the sealed chamber with an inert gas by repeating a gas exchangingoperation in which a step of exhausting the impurity gas from the sealedchamber through the gas exhausting system and a step of supplying theinert gas through the gas supplying system to the sealed chamber aresuccessively conducted.

[0020] In the present invention, it is more preferred that when the gasin the sealed chamber is exchanged with the predetermined gas, thecontrol system enable the gas exhausting system to exhaust the gas inthe sealed chamber and the gas supplying system to supply thepredetermined gas to the sealed chamber, while maintaining an amount ofchange in the pressure in the sealed chamber detected by the pressuresensor within a predetermined allowable range. For example, an impuritygas in the sealed chamber may be exchanged with an inert gas byexhausting the impurity gas in an extremely small amount from the sealedchamber through the gas exhausting system and subsequently, supplyingthe inert gas in an amount equal to the amount of exhausted impurity gasthrough the gas supplying system to the sealed chamber so that a radicalchange in pressure in the sealed chamber can be suppressed. By thisarrangement, stresses acting on lenses in contact with the gas in thesealed chamber become low, so that deterioration in performance of theillumination optical system can be avoided.

[0021] When the gas in the sealed chamber is exchanged with thepredetermined gas with high frequency during assembly of the exposureapparatus, while suppressing a radical change in pressure in the sealedchamber, solidification of sealing materials (or adhesives) used in thelenses in contact with the gas in the sealed chamber is promoted, sothat the time required for assembling the exposure-apparatus can bereduced. In this case, because the gas exchange is conducted in asubstantially stationary state with respect to the pressure in thesealed chamber, stresses acting on support members for supporting thelenses in contact with the gas in the sealed chamber are low and hence,damage to the lenses and deformation of the support members can beprevented. After operation of the exposure apparatus is started, it ispreferred to exchange the gas in the sealed chamber periodically duringidling of the exposure apparatus.

[0022] Further, in the present invention, it is preferred that theillumination optical system in the exposure apparatus comprise a lightsource adapted to emit exposure light, a shaping optical system adaptedto shape the exposure light passing therethrough and an opticalintegrator adapted to enable the exposure light to have a uniformilluminance distribution after passing through the shaping opticalsystem, and the sealed chamber be defined by two optical membersconstituting the shaping optical system. Because exposure light exhibitsconsiderably high illuminance on a plane of incidence of the opticalintegrator, deposition of haze substance on the surfaces of lenses inthe shaping optical system is likely to occur. However, by providing asealed chamber defined by lenses in the shaping optical system andexchanging an impurity gas in the sealed chamber with a predeterminedgas, the above-mentioned deposition of haze substance on the surfaces oflenses in the shaping optical system can be avoided.

[0023] Further, according to the present invention, there is provided amethod for conducting an exchange of gases in a sealed chamber providedin an exposure apparatus, comprising a gas exchanging step including:

[0024] a first sub-step of exhausting a gas in the sealed chamber from agas exhaust side thereof, while the sealed chamber is closed on a gassupply side thereof; and

[0025] a second sub-step of supplying an inert gas to the sealed chamberfrom the gas supply side thereof, while the sealed chamber is closed onthe gas exhaust side thereof,

[0026] wherein each of the first sub-step and the second sub-step isconducted in a substantially stationary state with respect to a pressureof the gas in the sealed chamber.

[0027] Still further, according to the present invention, there isprovided an exposure apparatus comprising:

[0028] an illumination optical system adapted to emit exposure light,the exposure light being adapted to illuminate a mask pattern to therebytransfer the mask pattern to a substrate;

[0029] a sealed chamber provided in the illumination optical system; and

[0030] a gas exchanging device adapted to exhaust a gas in the sealedchamber and supply an inert gas to the sealed chamber.

[0031] Still further, according to the present invention, there isprovided a projection exposure apparatus for transferring a pattern on amask to a photosensitive substrate, comprising:

[0032] a light source adapted to emit exposure light having a wavelengthrange in which a photosensitive substrate is sensitive to the exposurelight;

[0033] an illumination optical system provided between the light sourceand the mask;

[0034] a projection optical system provided between the mask and thephotosensitive substrate;

[0035] a sealed chamber containing a gas and provided in an optical pathof the exposure light between the light source and the photosensitivesubstrate; and

[0036] a gas circulating device connected to the sealed chamber,

[0037] the gas circulating device being adapted to exhaust the gascontained in the sealed chamber to an outside thereof, to therebycompensate for variations in intensity of the exposure light on thephotosensitive substrate.

[0038] Still further, according to the present invention, there isprovided a projection exposure apparatus for transferring a pattern on amask to a photosensitive substrate, comprising:

[0039] a light source adapted to emit exposure light having a wavelengthrange in which a photosensitive substrate is sensitive to the exposurelight;

[0040] a sealed chamber provided in an optical path of the exposurelight between the light source and the photosensitive substrate; and

[0041] a gas circulating device having a sensor and connected to thesealed chamber,

[0042] the sensor being adapted to detect and output informationcorresponding to a pressure in the sealed chamber, and

[0043] the gas circulating device being adapted to supply an inert gasto the sealed chamber in accordance with the information outputted fromthe sensor.

[0044] Still further, according to the present invention, there isprovided a method for transferring a pattern on a mask to aphotosensitive substrate, comprising the steps of:

[0045] illuminating the mask with exposure light, to thereby transferthe pattern on the mask to a photosensitive substrate; and

[0046] exchanging an inert gas contained in a sealed chamber withanother gas,

[0047] the sealed chamber being provided in an optical path of theexposure light,

[0048] to thereby compensate for variations in light transmittance andlight reflectance of an optical member provided in the optical path ofthe exposure light.

[0049] Still further, according to the present invention, there isprovided an exposure apparatus for transferring a pattern on a mask to aphotosensitive substrate, comprising:

[0050] an optical system adapted to allow exposure light to enter,

[0051] said exposure light being adapted to be irradiated to aphotosensitive substrate; and

[0052] a device adapted to supply a gas capable of suppressingattenuation of said exposure light to said optical system, according toa change in light transmittance of said optical-system which occurs dueto entrance of said exposure light.

[0053] Still further, according to the present invention, there isprovided a method for making an apparatus for transferring a pattern ona mask to a photosensitive substrate, comprising the steps of:

[0054] providing an optical system between a light source and aphotosensitive substrate,

[0055] said light source being adapted to emit exposure light,

[0056] said exposure light being adapted to enter said optical systemand irradiate said photosensitive substrate; and

[0057] providing a device adapted to supply a gas capable of suppressingattenuation of said exposure light to said optical system, according toa change in light transmittance of said optical system.

[0058] The foregoing and other objects, features and advantages of thepresent invention will be apparent from the following detaileddescription and appended claims taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0059]FIG. 1 is a perspective view of a partially cut-away exposureapparatus according to one embodiment of the present invention.

[0060]FIG. 2 is a cross-sectional view of a construction including aninput lens system ILS and a gas exchange mechanism in the exposureapparatus of FIG. 1.

[0061]FIG. 3 is a graph showing one example of a change in pressure inthe first sealed chamber 33A shown in FIG. 2 where a gas exchanging stepis repeatedly conducted.

[0062]FIG. 4 is a graph showing a relationship between the number oftimes the gas exchanging step is conducted and the concentration ofimpurity gas, with respect to the first sealed chamber 33A in FIG. 2where the gas exchanging step is repeatedly conducted.

DETAILED DESCRIPTION OF THE INVENTION

[0063] Hereinbelow, an exposure apparatus according to an embodiment ofthe present invention is explained, with reference to the drawings.

[0064]FIG. 1 shows a projection exposure apparatus as an exposureapparatus according to an embodiment of the present invention. In FIG.1, illumination light IL1 from exposure light source 1 comprising asuper-high pressure mercury-vapor lamp is collected by an ellipticmirror 2 and reflected by a mirror 3 and a mirror 4 toward a shutter 5.The shutter 5 is rotated by a drive motor 6, thereby opening and closinga passage for the illumination light IL1. When the shutter 5 is in anopen state, the illumination light IL1 passes through the shutter 5, andillumination light exclusive of an i-line is removed by an interferencefilter 7. The i-line which has passed through the interference filter 7constitutes exposure light IL and is reflected by a mirror 8 disposed soas to bend an optical path of the exposure light IL. The exposure lightIL then passes through an input lens system ILS comprising a first lens9A, a second lens 9B and a third lens 9C, and enters a fly-eye lens 10in the form of a substantially parallel beam. Incidentally, as theexposure light IL, an h-line (wavelength: 405 nm) or a g-line(wavelength: 436 nm) may be used, instead of the i-line. An excimerlaser beam, such as a KrF excimer laser beam and an ArF excimer laserbeam, an F₂ laser beam (wavelength: 157 nm) or a harmonic component of aYAG laser beam, may also be used as the exposure light IL.

[0065] An aperture stop plate 11 for an illumination system is rotatablyprovided on a plane of exit of the fly-eye lens 10. The aperture stopplate 11 includes a normal circular aperture stop 13A, an aperture stop13B for a modified light source, which comprises a plurality of smalleccentric apertures, an annular stop 13C and the like. These aperturestops are formed around a rotation shaft of the aperture stop plate 11.A desired aperture stop for an illumination system can be disposed onthe plane of exit of the fly-eye lens 10 by rotating the aperture stopplate 11 using a drive motor 12. A part of-the exposure light IL whichhas passed through the desired aperture stop on the plane of exit of thefly-eye lens 10 is reflected by a beam splitter 14 and enters anintegrator sensor 16 comprising a photoelectric conversion devicethrough a collective lens 15. Illuminance of the exposure light IL on awafer W can be indirectly monitored, based on a detection signalsupplied from the integrator sensor 16.

[0066] On the other hand, the exposure light IL which has passed throughthe beam splitter 14 passes through a first relay lens 17A, a projectiontype reticle blind (variable field stop) 18, a second relay lens 17B, amirror 19 disposed so as to bend the optical path of the exposure lightIL and a condenser lens 20, and illuminates a reticle R. Thus, anillumination optical system is constituted by the exposure light source1, the condenser lens 20 and the elements 2 to 19 provided between theexposure light source 1 and the condenser lens 20. Using the exposurelight IL passing through this illumination optical system, an image of apattern 21 on the reticle R is projected through a projection opticalsystem PL to the photoresist-coated wafer W.

[0067] In FIG. 1, a Z-axis is taken in a direction parallel to anoptical axis AX of the projection optical system PL and a coordinatesystem defined by an X-axis and a Y-axis which is perpendicular to theX-axis is contained in a plane perpendicular to the Z-axis. The reticleR is held on a reticle stage 28 which is adapted to perform alignment ofthe reticle R in an X direction, a Y direction and a rotation direction.The wafer W is held on a wafer holder 22 by suction. The wafer holder 22is fixedly provided on a wafer stage 23. The wafer stage 23 is adaptedto adjust the position of the wafer W along the Z-axis and a tilt angleof the wafer W so that the surface of the wafer W coincides with animage plane of the projection optical system PL. The wafer stage 23 isalso adapted to perform stepping of the wafer W in the X direction andthe Y direction and alignment of the wafer W. After exposure of one shotarea on the wafer W is finished, stepping of the wafer stage 23 isconducted to thereby move another shot area on the wafer W which issubsequently subjected to exposure to an exposure field of theprojection optical system PL, and exposure is conducted. Exposure isrepeatedly conducted in a manner such as mentioned above by a so-calledstep-and-repeat exposure method, to thereby conduct exposure of aplurality of shot areas on the wafer W.

[0068] In the projection exposure apparatus in this embodiment of thepresent invention, ultraviolet light is used as the exposure light IL.Therefore, ozone is produced, in the presence of the exposure light IL,from oxygen within ambient air. The ozone thus produced oxidizes anorganosilicon gas generated from sealing materials (or adhesives) usedin optical members, such as lenses and mirrors, and deposition of hazesubstance, such as silicon dioxide (SiO₂), on the surfaces of opticalmembers is likely to occur. In the present invention, in order toprevent such deposition of haze substance, a sealed chamber is providedin the optical path of the exposure light IL in the illumination opticalsystem and a gas in the sealed chamber is exchanged with a predeterminedgas. In the illumination optical system, the illuminance of the exposurelight IL is especially high in the optical path from the exposure lightsource 1 to the fly-eye lens 10 as an optical integrator. In thisembodiment, the sealed chamber is defined by adjacent lensesconstituting the input lens system ILS which is provided on the plane ofincidence of the fly-eye lens 10.

[0069]FIG. 2 shows a construction including the input lens system ILSand a gas exchange mechanism for the input lens system ILS. In FIG. 2,the first lens 9A, the second lens 9B and the third lens 9C aresuccessively provided along the optical path of the exposure light IL.The first lens 9A is fixed to a ring-shaped first lens barrel 31A with asealing material 45A being provided therebetween. A second lens barrel31B is disposed on a ring-shaped spacer 32A above the first lens barrel31A. The second lens 9B is fixed to the second lens barrel 31B with asealing material 45B being provided therebetween. A third lens barrel31C is disposed on a spacer 32B above the second lens barrel 31B. Thethird lens 9C is fixed to the third lens barrel 31C with a sealingmaterial 45C being provided therebetween. Each of the sealing materials45A to 45C contains silicon and also serves as an adhesive. In thisembodiment, two sealed chambers, namely, a first sealed chamber 33A anda second sealed chamber 33B are provided in the input lens system ILS.The first sealed chamber 33A is defined by the lenses 9A and 9B, thelens barrels 31A and 31B and the spacer 32A and shielded from a gassurrounding the first sealed chamber 33A. The second sealed chamber 33Bis defined by the lenses 9B and 9C, the lens barrels 31B and 31C and thespacer 32B and shielded from a gas surrounding the second sealed chamber33B. The lens barrels 31A to 31C and the spacers 32A and 32B are firmlyfixed so as not to allow the lenses 9A to 9C to be displaced due to achange in atmospheric pressure. Further, in order to maintain thetemperature of a gas in each of the sealed chambers 33A and 33B at apredetermined level, pipes 32C and 32D, each of which allows a fluidhaving a temperature controlled to a predetermined level to passtherethrough, are disposed on respective outer surfaces of the spacers32A and 32B.

[0070] A gas bomb 34 in which a high-purity nitrogen gas as an inert gasis sealably contained under high pressure is provided outside a chamberaccommodating the projection exposure apparatus. The high-puritynitrogen gas in the gas bomb 34 is supplied through an electromagneticvalve 35, a chemical filter 36 and an HEPA filter (high efficiencyparticulate air-filter) 37 to a gas supply pipe 38. The opening andclosing of the electromagnetic valve 35 is controlled by a pressurecontrol system 40 comprising a computer. A first pipe 38 a branched fromthe gas supply pipe 38 is connected to the first sealed chamber 33Athrough an electromagnetic valve 39A provided with a check valve. Asecond pipe 38 b branched from the gas supply pipe 38 is connected tothe second sealed chamber 33B through an electromagnetic valve 39Bprovided with a check valve. The opening and closing of each of theelectromagnetic valves 39A and 39B is also controlled by the pressurecontrol system 40.

[0071] Further, the first sealed chamber 33A is connected to a gasexhaust pipe 41 through a first pipe 41 a for gas exhaustion and anelectromagnetic valve 42A provided with a check valve. The second sealedchamber 33B is connected to the gas exhaust pipe 41 through a secondpipe 41 b for gas exhaustion and an electromagnetic valve 42B providedwith a check valve. The gas exhaust pipe 41 opens to the atmosphereoutside the chamber accommodating the projection exposure apparatusthrough a gas exhaust pump 43 and a filter device (not shown). Theopening and closing of each of the electromagnetic valves 42A and 42Band the operation of the gas exhaust pump 43 are controlled by thepressure control system 40. Pressure sensors 44A and 44B are provided inthe sealed chambers 33A and 33B, respectively, so as to detect pressuresin the sealed chambers 33A and 33B. Detection signals are supplied fromthe pressure sensors 44A and 44B to the pressure control system 40.Thus, the pressure control system 40 monitors respective pressures ofgasses in the sealed chambers 33A and 33B, based on the detectionsignals from the pressure sensors 44A and 44B.

[0072] Basically, the gas exchange mechanism shown in FIG. 2 is operatedas follows. Initially, while the electromagnetic valves 39A and 39B on agas supply side are closed, the pressure control system 40 opens theelectromagnetic valves 42A and 42B on a gas exhaust side, and actuatesthe gas exhaust pump 43 so that a part of the gas in the first sealedchamber 33A, that is, remaining oxygen and an impurity gas, such as anorganosilicon gas generated from the sealing materials 45A and 45B, anda part of the gas in the second sealed chamber 33B, that is, remainingoxygen and an impurity gas, such as an organosilicon gas generated fromthe sealing materials 45B and 45C, are exhausted. Subsequently, thepressure control system 40 closes the electromagnetic valves 42A and 42Bon the gas exhaust side and opens the electromagnetic valves 39A and 39Bon the gas supply side, and also opens the electromagnetic valve 35, tothereby supply the high-purity nitrogen gas from the gas bomb 34 throughthe gas supply pipe 38 to each of the sealed chambers 33A and 33B. Thepressure control system 40 ensures that the gas exchange is conducted ina substantially stationary state so that no radical changes occur withrespect to the pressures of gases in the sealed chambers 33A and 33B,which pressures are detected by the pressure sensors 44A and 44B,respectively. Thus, the respective amounts of oxygen and the impuritygas (such as an organosilicon gas generated from the sealing materials)in each of the sealed chambers 33A and 33B decrease, so that therespective concentrations of oxygen and impurities in the gas in each ofthe sealed chambers 33A and 33B become low and hence, a process ofdeposition of haze substance on each of the lenses 9A to 9C isinterrupted, to thereby prevent occurrence of haze on the surfaces ofthese lenses.

[0073] In this embodiment, the electromagnetic valves 39A, 39B, 42A and42B, each provided with a check valve, are employed. Therefore, thegasses in the sealed chambers 33A and 33B flow in a single directionfrom the gas bomb 34 toward the gas exhaust pump 43 without occurrenceof a reverse gas flow. Therefore, the impurity gas in each of the sealedchambers 33A and 33B can be efficiently exchanged with the high-puritynitrogen gas.

[0074] Next, referring to FIGS. 1 to 4, explanation is made on oneexample of an operation for conducting an exchange of gases in the firstsealed chamber 33A in a substantially stationary state using the gasexchange mechanism shown in FIG. 2. This operation is mainly conductedduring idling of the projection exposure apparatus between exposureoperations.

[0075] When the pressure P of the gas in the first sealed chamber 33A ata time point t₀, when an exchange of gases is started is indicated as aninitial value P₀, this initial value P₀ is substantially equal to thepressure of a gas surrounding the illumination optical system (1 atm inthis embodiment). At the time point t₀, the electromagnetic valve 42A onthe gas exhaust side is opened while the electromagnetic valve 39A onthe gas supply side is closed, and the gas exhaust pump 43 is actuatedso as to exhaust the gas in the first sealed chamber 33A until thepressure P of the gas in the first sealed chamber 33A, which is detectedby the pressure sensor 44A, decreases by an amount Δp which is within apredetermined allowable range. The time period between the time point t₀and the time point when the pressure P decreases by the allowable amountΔp is indicated as Δt₁.

[0076]FIG. 3 is a graph showing one example of a change in pressure inthe first sealed chamber 33A shown in FIG. 2 where the exchange of gasesis conducted. In the graph of FIG. 3, the change in the pressure P inthe first sealed chamber 33A is indicated by a solid curved line 51. InFIG. 3, the abscissa indicates the time t and the ordinate indicates thepressure P. In FIG. 3, the pressure P decreases by the allowable amountΔp from the initial value P₀ at a time point t₁. Therefore, in FIG. 3,the above-mentioned time period Δt₁ is indicated as the time periodbetween the time point to and the time point t₁.

[0077] Subsequently, the electromagnetic valve 42A on the gas exhaustside is closed and the electromagnetic valve 39A on the gas supply sideis opened. The electromagnetic valve 35 is also opened, to therebysupply the high-purity nitrogen gas from the gas bomb 34 to the firstsealed chamber 33A. In this instance, using the pressure sensor 44A, thehigh-purity nitrogen gas is supplied until the pressure P in the firstsealed chamber 33A is recovered to the initial value P₀. As indicated bythe solid curved line 51 in FIG. 3, the pressure P is recovered to theinitial value P₀ at a time point t₂. The time period between the timepoint t₁ and the time point t₂ is indicated as Δt₂. Thereafter, theabove-mentioned operation (hereinafter, frequently referred to simply as“gas exchanging step”) comprising a step of exhausting the gas in thefirst sealed chamber 33A until the pressure P of the gas in the firstsealed chamber 33A decreases by the allowable amount Δp from the initialvalue P₀ (first sub-step) and a step of supplying the high-puritynitrogen gas to the first sealed chamber 33A until the pressure P isrecovered to the initial value P₀ (second sub-step) is repeated. Whenthe gas exchanging step is repeated, the solid curved line 51 whichindicates the change in the pressure P exhibits a sine waveform.

[0078]FIG. 4 is a graph showing a relationship between the number n oftimes the gas exchanging step is conducted and the concentration C ofimpurity gas, with respect to the first sealed chamber 33A in FIG. 2where the gas exchanging step is repeatedly conducted. The graph of FIG.4 is obtained in a manner as mentioned below. When the amount of gas inthe first sealed chamber 33A which is exchanged at each gas exchangingstep (hereinafter, frequently referred to simply as “gas exchangeamount”) is indicated as Δq, the value of Δq is determined in accordancewith the above-mentioned allowable amount Δp. Further, when the internalvolume of the first sealed chamber 33A is indicated as Q and theconcentration of impurity gas (such as an organosilicon gas) in the gasin the first sealed chamber 33A after the gas exchanging step isconducted at i time(s) (i=1, 2, . . . ) is indicated as C_(i), becausegases become mixed at a sufficiently high rate, the concentrationC_(i+1) of impurity gas after the gas exchanging step is conducted (i+1)times is determined in accordance with the following formula (1).

C _(i+1) =C _(i)(1−Δq/Q)  (1)

[0079] From this formula (1), the concentration C_(i) of impurity gas isindicated by the following formula (2), using the concentration C₁ ofimpurity gas after the gas exchanging step is conducted at one time.

C _(i) =C ₁(1−Δq/Q)¹⁻¹  (2)

[0080] When a target value of the concentration C of impurity gas isindicated as C_(L), the following formula (3) is obtained from theformula (2), with respect to the number N of times the gas exchangingstep needs to be conducted for achieving the target value C_(L).

C _(N) =C _(i)(1−Δq/Q)^(N−1) ≦C _(L)  (3)

[0081] Accordingly, with respect to the number n (n=1, 2, . . . , N) oftimes the gas exchanging step is conducted, the concentration C ofimpurity gas in the gas in the first sealed chamber 33A changes asindicated by a solid curved line 52 in the graph of FIG. 4. On the otherhand, the formula (3) can be reformulated as follows.

(1−Δq/Q)^(N−1) ≦C _(L) /C ₁  (4A)

(N−1)log(1−Δq/Q)≦log(C _(L) /C ₁)  (4B)

[0082] With respect to the formula (4B), log (1−Δq/Q)<0 and log (C _(L)/C ₁)<0. Therefore, the formula (4B) can be reformulated as follows.

(N−1)≧log(C _(L) /C ₁)log(1−Δq/Q)  (4C)

N≧1+log(C _(L) /C ₁)/log(1−Δq/Q)  (4D)

[0083] Therefore, when the internal volume Q of the first sealed chamber33A, an appropriate gas exchange amount Δq (or the allowable amount Δpof change in pressure of the gas in the first sealed chamber 33A), theconcentration C₁ of impurity gas after the gas exchanging step isconducted at one time and the target value C_(L) of the concentration Cof impurity gas are determined, the minimum value N_(min) of the numberN of times the gas exchanging step needs to be conducted for suppressingthe concentration C of impurity gas to the target value C_(L) or lesscan be determined, in accordance with the formula (4D). In thisembodiment, the number of times the gas exchanging step is conducted isN_(min) which is the minimum value of the integer N satisfying theformula (4D). Thus, the concentration C of impurity gas can besuppressed to the target value C_(L) or less by conducting the gasexchanging step at N_(min) time(s).

[0084] With respect to the gas exchange amount Δq (or the allowableamount Δp of change in pressure of the gas in the first sealed chamber33A), when the gas exchange amount Δq is too large, stresses acting onthe lenses, such as the first lens 9A in FIG. 2, become high, so thatproblems arise, such as displacement, a change in aberration, damage andbreakage of lenses. Even when damage or breakage of lenses is avoided, asubstantial amount of stress is likely to have an adverse effect on theaberration of lenses which has already been corrected. Therefore, inthis embodiment of the present invention, the gas exchange amount Δq (orthe allowable amount Δp of change in pressure of the gas in the firstsealed chamber 33A) is suppressed to a level such that the pressure inthe first sealed chamber 33A changes in a substantially stationarystate. For example, the allowable amount Δp is determined as being anamount several times the amount which is capable of being detected bythe pressure sensor 44A in the first sealed chamber 33A in FIG. 2, andthe gas exchange amount Δq is determined from the thus determined amountΔp. By this arrangement, an undesirable increase in stress acting onlenses during the exchange of gases in the first sealed chamber 33A canbe prevented and the above-mentioned problems accompanying the exchangeof gases, such as a change in aberration of lenses, can be suppressedwithin a sufficiently narrow allowable range.

[0085] When the minimum value N_(min) of the number N of times the gasexchanging step is conducted, which is determined in accordance with theformula (4D), becomes large so that a total gas exchange time [i.e., thetime period during which the gas exchanging step is conducted at N_(min)time(s)] exceeds an idling time of the projection exposure apparatus, atime for conducting the gas exchanging step at one time may be reducedby reducing the time period t₁ and the time period t₂ in FIG. 3. Withrespect to the second sealed chamber 33B in FIG. 2, the impurity gas inthe second sealed chamber 33B is exchanged with the inert gas insubstantially the same manner as in the first sealed chamber 33A, whilesuppressing a change in aberration of lenses and the like.

[0086] Preferably, the above-mentioned gas exchanging step is conductedperiodically during idling of the projection exposure apparatus, becausean organosilicon gas is gradually generated from the sealing materials45A to 45C in FIG. 2. By this arrangement, gradual deposition of hazesubstance on the surfaces of lenses can be prevented.

[0087] Although the gas exchanging step is conducted during idling ofthe projection exposure apparatus in the above-mentioned embodiment, inthe present invention, the gas exchanging step may be conducted duringassembly and adjustment of the projection exposure apparatus in a manneras mentioned below. That is, immediately after the lenses 9A to 9C arefixed to the lens barrels 31A to 31C with the sealing materials 45A to45C being provided therebetween, the gas in each of the sealed chambers33A and 33B may be exchanged with a high-purity nitrogen gas through thegas exchange mechanism in FIG. 2, while maintaining an amount of changein pressure in each of the sealed chambers 33A and 33B at Δp or less. Inthis instance, because the amount of change in pressure during the gasexchanging step is as small as Δp or less, it is unnecessary to waituntil the sealing materials 45A to 45C are completely solidified.Further, because the organosilicon gas generated from the sealingmaterials 45A to 45C during solidification thereof is efficientlyexhausted through the gas exhaust pump 43, the solidification time canbe reduced and the time for assembly can also be reduced. Further, thereis no phenomenon such that organosilicon substance adheres to andremains on the surfaces of the lenses 9A to 9C and the inner walls ofthe lens barrels 31A and 31C. Thus, deposition of haze substance on thelenses 9A to 9C can be completely prevented.

[0088] In the above-mentioned embodiment, when moisture remains in thegas supply pipe 38 in FIG. 2, such moisture enters the sealed chambers33A and 33B, in accordance with the flow of gas supplied to the sealedchambers 33A and 33B. In this case, not only does a lowering ofefficiency in exhausting impurities occur, but also the moisture reactwith coating materials on lenses in an early stage and the resultantreaction product is likely to be deposited on the surfaces of lenses,thereby contaminating the lenses. Therefore, it is preferred to providethe gas supply pipe 38 with another exhaust port and preliminarily cleanthe gas supply pipe 38 by flowing an inert gas, such as a nitrogen gas(N₂) and helium (He), from this exhaust port through the gas supply pipe38.

[0089] In the above-mentioned embodiment, the gas exchange mechanism isprovided in the input lens system ILS in FIG. 1. However, in the presentinvention, the gas exchange mechanism may also be applied to, forexample, the interference filter 7, the fly-eye lens 10, the beamsplitter 14, the relay lenses 17A and 17B in the illumination opticalsystem, in order to prevent deposition of haze substance on theseoptical members. Further, when two fly-eye lenses 10 are provided so asto improve uniformity of illuminance distribution of exposure light anda relay lens system is provided between these two fly-eye lenses, thegas exchange mechanism may be provided in this relay lens system.

[0090] Further, as the inert gas used in the gas exchange mechanism, ahigh-purity nitrogen gas is employed in the above-mentioned embodiment.However, in the present invention, a chemically stable gas, for example,a rare gas, such as helium or hydrogen, may also be used as the inertgas. In an exposure apparatus in which a KrF excimer laser is used,dried air which is chemically clean may be used as the inert gas. Theabove-mentioned dried air is obtained by passing air through a chemicalfilter and adjusting the humidity of the filtered air to, for example,about 5% or less. With respect to the gas bomb 34 connected through thegas supply pipe 38 to the electromagnetic valves 39A and 39B on the gassupply side and the gas exhaust pump 43 connected through the gasexhaust pipe 41 to the electromagnetic valves 42A and 42B on the gasexhaust side in FIG. 2, the gas bomb 34 and the gas exhaust pump 43 maybe temporarily connected only when the gas exchanging step is conductedin each of the sealed chambers 33A and 33B. That is, the sealed chambers33A and 33B may be arranged so as to have a construction which iscapable of being connected to the gas bomb 34 and the gas exhaust pump43 for conducting the gas exchanging step.

[0091] Incidentally, when the pressure on the gas supply side is set toa level such that no reverse gas flow occurs, as each of theelectromagnetic valves 39A, 39B, 42A and 42B, a simple electromagneticvalve may be used, instead of the electromagnetic valve provided withthe check valve.

[0092] Generally, in the projection exposure apparatus, in order tocorrect variations in image-forming characteristics, such as themagnification and distortion of the projection optical system, which arecaused by a change in atmospheric pressure, and variations in theseimage-forming characteristics which are caused by irradiation ofexposure light (so-called irradiation-dependent variations), sealedchambers are provided in the projection optical system at several siteswhere the above-mentioned variations in image-forming characteristicscan be effectively corrected and pressures in these sealed chambers areactively changed. Alternatively, the above-mentioned variations inimage-forming characteristics are corrected directly by controlling thepositions of lenses in the projection optical system (so-called lenscontrol). Especially, when the pressures in sealed chambers arecontrolled, bellows are generally used.

[0093] Therefore, in the above-mentioned embodiment of the presentinvention, a sealed chamber having a pressure which is capable of beingcontrolled may be provided in the projection optical system PL so thatthe sealed chamber contains the gas in a space between lenses which areuseful for effectively conducting correction of aberration in theprojection optical system PL. In this case, the pressure in the sealedchamber may be controlled utilizing a pressure of the high-puritynitrogen gas, in stead of using bellows. By this arrangement, not onlycan variations in image-forming characteristics be corrected, but alsodeposition of haze substance on the surfaces of lenses in contact withthe gas in the sealed chamber in the projection optical system PL can beprevented.

[0094] When the sealed chamber is provided in an optical path betweenthe reticle and the wafer, the pressure of the inert gas contained inthe sealed chamber may not be controlled, and as disclosed in, forexample, U.S. Pat. No. 5,117,255, optical characteristics (such as afocus position, a magnification, aberrations and telecentricity) andimage-forming characteristics (such as image contrast) with respect toan image of the pattern on the reticle may be adjusted by moving atleast one optical member in the projection optical system PL. In thiscase, an inert gas is selectively supplied to the sealed chamber so asto compensate for variations in light transmittance of the illuminationoptical system and/or the projection optical system PL, i.e., variationsin light intensity of exposure light on the wafer. With respect to theoptical integrator provided in the illumination optical system, theoptical integrator is not limited to the fly-eye lens. A rod integratormay also be used as the optical integrator. The fly-eye lens and the rodintegrator may be used in combination, as disclosed in U.S. Pat. No.4,918,583.

[0095] The present invention can be applied to not only a one-shotexposure type projection exposure apparatus, but also a scanningexposure type projection exposure apparatus, such as a step-and-scantype. The present invention can also be applied to an exposure apparatusof a proximity type or a contact type in which the projection opticalsystem is not used. Thus, the present invention is not limited to theabove-mentioned embodiment. Various modifications are possible withoutdeparting from the scope of the present invention as defined in theappended claims.

[0096] In the exposure apparatus of the present invention, a sealedchamber is provided in an optical path of exposure light in theillumination optical system. The gas in the sealed chamber isexchangeable. Therefore, for example, when the gas in the sealed chamberwhich is likely to generate haze substance is periodically exchangedwith another gas, deposition of substance which lowers lighttransmittance and light reflectance of optical members, such as lenses,in the illumination optical system is unlikely to occur. Therefore, alowering of illuminance of exposure light on the mask and a non-uniformdistribution with respect to illuminance of exposure light can besuppressed.

[0097] Further, in the exposure apparatus of the present invention inwhich a pressure sensor is provided in the sealed chamber, the pressurein the sealed chamber can be maintained at a desired level.

[0098] In this instance, it is preferred that when the gas is in thesealed chamber is exchanged with a predetermined gas, the gas in thesealed chamber be exhausted through the gas exhausting system and thepredetermined gas be supplied to the sealed chamber through the gassupplying system, while maintaining an amount of change in the pressurein the sealed chamber detected by the pressure sensor within apredetermined allowable range, from the viewpoint of suppressing aradical change in pressure in the sealed chamber. When a radical changein pressure in the sealed chamber is suppressed, stresses acting onoptical members (such as lenses) in contact with the gas in the sealedchamber become low, so that an adverse effect on aberration of theoptical members can be avoided.

[0099] Further, the present invention is especially advantageous whenthe illumination optical system comprises a light source adapted to emitexposure light, a shaping optical system adapted to shape the exposurelight passing therethrough and an optical integrator adapted to enablethe exposure light to have a uniform illuminance distribution afterpassing through the shaping optical system, and the sealed chamber isdefined by two optical members constituting the shaping optical system,because it is possible to exchange the gas in a region where theilluminance of exposure light is high and therefore deposition of hazesubstance is likely to occur, and prevent occurrence of haze in thatregion.

[0100] The entire disclosure of Japanese Patent Application No. Hei9-75355 filed on Mar. 27, 1997 is incorporated herein by reference inits entirety.

What is claimed is:
 1. An exposure apparatus for illuminating a patternon a mask with exposure light passing through an illumination opticalsystem, to thereby transfer said pattern on the mask to a substrate,comprising: a sealed chamber provided in an optical path of saidexposure light in said illumination optical system, said sealed chambercontaining a gas and shielded from a gas surrounding said sealed chamberin said illumination optical system; and a gas exchanging device adaptedto exchange the gas in said sealed chamber with a predetermined gas. 2.The exposure apparatus according to claim 1, wherein said gas exchangingdevice comprises: a gas exhausting system adapted to exhaust the gas insaid sealed chamber; a gas supplying system adapted to supply thepredetermined gas to said sealed chamber; a pressure sensor provided insaid sealed chamber to detect a pressure in said sealed chamber; and acontrol system adapted to control an operation of each of said gasexhausting system and said gas supplying system, based on said pressurein the sealed chamber detected by said pressure sensor, to therebyexchange the gas in said sealed chamber with the predetermined gas. 3.The exposure apparatus according to claim 2, wherein when the gas insaid sealed chamber is exchanged with the predetermined gas, saidcontrol system enables said gas exhausting system to exhaust the gas insaid sealed chamber and said gas supplying system to supply thepredetermined gas to said sealed chamber, while maintaining an amount ofchange in said pressure in the sealed chamber detected by said pressuresensor within a predetermined allowable range.
 4. The exposure apparatusaccording to claim 1, wherein said illumination optical systemcomprises: a light source adapted to emit exposure light; a shapingoptical system adapted to shape said exposure light passingtherethrough; and an optical integrator adapted to enable said exposurelight to have a uniform illuminance distribution after passing throughsaid shaping optical system, and said sealed chamber is defined by twooptical members constituting said shaping optical system.
 5. Theexposure apparatus according to claim 1, wherein an inert gas iscontained in said sealed chamber.
 6. The exposure apparatus according toclaim 1, wherein said sealed chamber is provided at a position at whichsaid exposure light exhibits high illuminance.
 7. The exposure apparatusaccording to claim 1, wherein said illumination optical system includesan input lens system and said sealed chamber is provided in said inputlens system.
 8. The exposure apparatus according to claim 1, furthercomprising a device adapted to maintain a temperature of the gas in saidsealed chamber at a predetermined level.
 9. The exposure apparatusaccording to claim 1, wherein the gas in said sealed chamber isexchanged periodically during idling of said exposure apparatus.
 10. Theexposure apparatus according to claim 1, wherein the gas in said sealedchamber is exchanged during assembly and adjustment of said exposureapparatus.
 11. A method for conducting an exchange of gases in a sealedchamber provided in an exposure apparatus, comprising a gas exchangingstep including: a first sub-step of exhausting a gas in said sealedchamber from a gas exhaust side thereof, while said sealed chamber isclosed on a gas supply side thereof; and a second sub-step of supplyingan inert gas to said sealed chamber from the gas supply side thereof,while said sealed chamber is closed on the gas exhaust side thereof,wherein each of said first sub-step and said second sub-step isconducted in a substantially stationary state with respect to a pressureof the gas in said sealed chamber.
 12. The method according to claim 11,wherein said exchange of gases in said sealed chamber is conductedperiodically during idling of said exposure apparatus.
 13. The methodaccording to claim 11, wherein said exchange of gases in said sealedchamber is conducted during assembly and adjustment of said exposureapparatus.
 14. The method according to claim 11, wherein in said firstsub-step, a part of the gas in said sealed chamber is exhausted from thegas exhaust side of said sealed chamber and said gas exchanging step isconducted at least twice, and said method further comprises a step ofmeasuring a total gas exchange time by counting the number of times saidgas exchanging step is conducted.
 15. The method according to claim 14,wherein said exchange of gases is conducted periodically during idlingof said exposure apparatus and said method further comprises the stepsof: measuring an idling time of said exposure apparatus; and comparingsaid idling time with said total gas exchange time, and when said totalgas exchange time exceeds said idling time, a time for conducting saidgas exchanging step at one time is reduced.
 16. An exposure apparatuscomprising: an illumination optical system adapted to emit exposurelight, said exposure light being adapted to illuminate a mask pattern tothereby transfer said mask pattern to a substrate; a sealed chamberprovided in said illumination optical system; and a gas exchangingdevice adapted to exhaust a gas in said sealed chamber and supply aninert gas to said sealed chamber.
 17. The exposure apparatus accordingto claim 16, wherein said illumination optical system includes an inputlens system, said input lens system having at least one pair of lenses,and said sealed chamber is defined by said at least one pair of lenses.18. The exposure apparatus according to claim 17, wherein said gasexchanging device comprises: a gas exhausting system adapted to exhaustan impurity gas in said sealed chamber; a gas supplying system adaptedto supply an inert gas to said sealed chamber; and a control systemadapted to control said gas exhausting system and said gas supplyingsystem so that a change in pressure in said sealed chamber is within apredetermined range.
 19. The exposure apparatus according to claim 18,wherein said gas exhausting system includes: a gas exhaust pipe adaptedto exhaust said impurity gas from said sealed chamber; anelectromagnetic valve provided in said gas exhaust pipe to open andclose said gas exhaust pipe, said electromagnetic valve being providedwith a check valve; and a gas exhaust pump connected to a gas exhaustside of said electromagnetic valve in the gas exhaust pipe, wherein saidgas supplying system includes: a gas supply pipe adapted to supply saidinert gas to said sealed chamber; an electromagnetic valve provided insaid gas supply pipe to open and close said gas supply pipe, saidelectromagnetic valve being provided with a check valve; and a gas bombconnected to a gas supply side of said electromagnetic valve in the gassupply pipe, and wherein said control system is adapted to control saidelectromagnetic valve in the gas exhaust pipe and said electromagneticvalve in the gas supply pipe.
 20. A projection exposure apparatus fortransferring a pattern on a mask to a photosensitive substrate,comprising: a light source adapted to emit exposure light having awavelength range in which a photosensitive substrate is sensitive tosaid exposure light; an illumination optical system provided betweensaid light source and said mask; a projection optical system providedbetween said mask and said photosensitive substrate; a sealed chambercontaining a gas and provided in an optical path of said exposure lightbetween said light source and said photosensitive substrate; and a gascirculating device connected to said sealed chamber, said gascirculating device being adapted to exhaust the gas contained in saidsealed chamber to an outside thereof, to thereby compensate forvariations in intensity of said exposure light on said photosensitivesubstrate.
 21. The apparatus according to claim 20, wherein said sealedchamber is defined by at least two optical members provided in saidillumination optical system.
 22. The apparatus according to claim 20,wherein said gas circulating device is adapted to supply nitrogen orhelium to said sealed chamber.
 23. A projection exposure-apparatus fortransferring a pattern on a mask to a photosensitive substrate,comprising: a light source adapted to emit exposure light having awavelength range in which a photosensitive substrate is sensitive tosaid exposure light; a sealed chamber provided in an optical path ofsaid exposure light between said light source and said photosensitivesubstrate; and a gas circulating device having a sensor and connected tosaid sealed chamber, said sensor being adapted to detect and outputinformation corresponding to a pressure in said sealed chamber, and saidgas circulating device being adapted to supply an inert gas to saidsealed chamber in accordance with the information outputted from saidsensor.
 24. A method for transferring a pattern on a mask to aphotosensitive substrate, comprising the steps of: illuminating saidmask with exposure light, to thereby transfer said pattern on the maskto a photosensitive substrate; and exchanging an inert gas contained ina sealed chamber with another gas, said sealed chamber being provided inan optical path of said exposure light, to thereby compensate forvariations in light transmittance and light reflectance of an opticalmember provided in said optical path of the exposure light.
 25. Anexposure apparatus for transferring a pattern on a mask to aphotosensitive substrate, comprising: an optical system adapted to allowexposure light to enter, said exposure light being adapted to beirradiated to a photosensitive substrate; and a device adapted to supplya gas capable of suppressing attenuation of said exposure light to saidoptical system, according to a change in light transmittance of saidoptical system which occurs due to entrance of said exposure light. 26.A method for making an apparatus for transferring a pattern on a mask toa photosensitive substrate, comprising the steps of: providing anoptical system between a light source and a photosensitive substrate,said light source being adapted to emit exposure light, said exposurelight being adapted to enter said optical system and irradiate saidphotosensitive substrate; and providing a device adapted to supply a gascapable of suppressing attenuation of said exposure light to saidoptical system, according to a change in light transmittance of saidoptical system.